Method of analyzing uranium and thorium ores



July 18, 1961 J, E o 2,993,122

METHOD OF ANALYZING URANIUM AND THORIUM ORES Filed. Dec. 23, 1957 10Sheets-Sheet 1 FIGURE 1 THE GAMMA SPECTRUM OF AN EQUILIBRIUM URANIUM OREGAMMA-RAY INTENSITY GAMMA-RAY ENERGY (Mev) INVENTOR JOHN L. MERO ATTO RN EY5 y 1961 J. MERO 2,993,122

METHOD OF ANALYZING URANIUM AND THORIUM ORES Filed Dec. 23, 1957 10Sheets-Sheet 2 FIGURE 2 THE GAMMA-RAY SPECTRUM OF A URANIUM-RICHPLEISTOCENE GRAVEL URANIUM ORE DEPOSIT. THE GAMlVLA-RAY SPECTRA OFCHEMICALLY REFINED URANIUM SALTS ARE SIMILAR TO THIS GRAPH.

GAMMA-RAY INTENSITY GAMMA-RAY ENERGY (MeV) INVENTOR JOHN L. MERO ATTORNEYS July 18, 1961 J.'L. MERO 2,993,122

METHOD OF ANALYZING URANIUM AND THORIUM ORES Filed Dec. 23, 1957 10Sheets-Sheet 3 FIGURE 3 THE GAMMA-RAY SPECTRUM OF A RADIUM-RICH oRE.THIS SPECIFIC GRAPH 1s OF A SULPHATE SCALE FOUND ON A WATER TROUGH NEARA WATER WELL. THE NUCLIDES CAUSING THE PEAKS IN THIS GRAPH AREPRINCIPALLY E1 AND Pb GAMMA-RAY INTENSITY GAMMA-RAY ENERGY (MeV)INVENTOR JOHN L. MERO ATTORN EY$ July 18, 1961 J. L. MERO 2,993,122

METHOD OF ANALYZING URANIUM AND THORIUM ORES Filed Dec. 25, 1957 10Sheets-Sheet 4 FIGURE 4 THE GAMMA-RAY SPECTRUM OF A URANIUM-DEFICIENTORE. SOME OF THE URANIUM GROUP OF NUCLIDES OF THE URANIUM DECAY SERIESARE STILL ACTIVE AS EVIDENCED BY THE LARGE COUNT OF CHANNEL 1. TAILINGSFROM URANIUM CONCENTRATION PROCESSES HAVE A SIMILAR SPECTRUM.

GAMMA-RAY INTENSITY GAMMA-RAY ENERGY (MeV) INVENTOR JOHN L. MERO ATTORNEY$ July 18, 1961 J. MERO 2,993,122

METHOD OF ANALYZING URANIUM AND THORIUM ORES Filed Dec. 25, 1957 10Sheets-Sheet 5 FIGURE 53: THE GAMMA-RAY SPECTRUM OF THE RADIUM GROUP OFNUCLIDES.

I n I I FIGURE 5b: THE GAMMA-RAY SPECTRUM OF THE URANIUM GROUP OFNUCLIDES.

FIGURE 5c: THE GAMMA-RAY SPECTRUM OF THE COMBINED URANIUM AND RADIUMGROUPS FORMING AN EQUILIBRIUM ORE.

GAMMA-RAY ENERGY (Mev) INV EN TOR.

JOHN L. MERO fi/zww 6M ly 18, 1961 .1. MERO 2,993,122

METHOD OF ANALYZING URANIUM AND THORIUM ORES Filed Dec. 23, 1957 10Sheets-Sheet 6 FIGURE 6 THE GAMMA SPECTRUM OF AN EQUILIBRIUM URANIUM OREWITH THE CHANNELS USED IN ANALYSIS INDICATED.

GAMMArRAY INTE NSITY GAMMA-RAY ENERGY (MeV) INVENTOR.

JOHN L. MERO July 18, 1961 J. L. MERO 2,993,122

METHOD OF ANALYZING URANIUM AND THORIUM ORES Filed Dec. 23, 1957 1OSheets-Sheet 7 FIGURE 7 THE GAMMA-RAY SPECTRUM OF AN EQUILIBRIUM THoRIUMoRE WITH THE CHANNELS USED IN oRE ANALYSIS INDICATED. THERE IS SOMEURANIUM IN THIS on. THE on IS ABOUT 20 PARTS THoRIUM TO 1 PART URANIUM.

GAMMA-RAY INTENSITY GAMMA-RAY ENERGY (MeV) INVENTOR.

JOHN L. MERO' BY 73W s M July 18, 1961 J. L. MERO 2,993,122

METHOD OF ANALYZING URANIUM AND THORIUM ORES Filed Dec. 25, 1957 10Sheets-Sheet 8 FIGURE 8 THE GAMMA-RAY SPECTRUM OF A CHEMICALLY REFINEDTHORIUM OXIDE THAT IS SEVERAL YEARS OLD. A COMPARISON OF THIS GRAPH WITHTHE GRAPH OF AN EQUILIBRIUM THORIUM ORE SUCH AS THAT ILLUSTRATED INFIGURE 7 INDICATES THAT THORIUM WILL ATTALN A STATE OF NEAR EQUILIBRIUMVERY RAPIDLY.

a GAMMA-RAY INTENSITY GAMMA-RAY'ZENERGY (MeV) INVENTOR JOHN L. MEROATTORN EY5 July 18, 1961- J. MERO 2,993,122

METHOD OF ANALYZING URANIUM AND THORIUM ORES Filed Dec. 25, 1957 10Sheets-Sheet 9 FIGURE 9 THE GAMMA-RAY SPECTRUM OF A SAMPLE OF MATERIALFROM A DEPOSIT OF THE RADIUM NUCLIDES AND THEIR DAUGHTERS OF BOTH THEURANIUM AND THE THORIUM DECAY SERIES. THE MOTHER NUCLIDES, U238 AND ThARE ABSENT FROM THIS MATERIAL.

GAMMA-RAY INTENSITY GAMMA-:RAY ENERGY (MeV) INVENTOR JOHN L. MERO %m& r544% ATTO R N EYS July 18, 1961 J. L. MERO METHOD OF ANALYZIN GURANIUMAND THORIUM ORES Filed Dec. 23, 1957 10 Sheets-Sheet 10 FIGURE 1 0 APLOT OF THE CHANNEL 3-CHANNEL 4 COUNT RATE RATIO VERSUS THE RATIO OF THEPERCENT U 0 TO THE PERCENT Th0 IN COMPOSITE ORES.

htx. OHHJYM EDHNOHHF EDHQMD 6 5 A: 3 2 1 oo 0 0n nw O on 0 0 CHANNELB-CHANNEL 4 RATIO, c /c United States Patent O 2,993,122 METHOD OFANALYZING URANIUM AND THORIUM ORES John L. Mero, 1823 Spruce St.,Berkeley 9, Calif. Filed Dec. 23, 1957, Ser. No. 704,490 4 Claims. (Cl.250-833) My invention relates to ore assaying and more particularly to aradiometric method of determining the equilibrium condition of naturallyoccurring radioactive arm, and determining ore grades.

Among the objects of my invention are:

(1) To provide a novel and improved method of reliably assaying uraniumand thorium ores;

(2) To provide a novel and improved radiometric method for uranium andthorium ores;

(3) To provide a novel and improved method for analyzing uranium andthorium ores, which method is relatively simple, reliable and applicableto automatic routine procedure;

(4) To provide a novel and improved method which enables one todetermine Whether a specific uranium or thorium ore is in a state ofradioactive equilibrium or not;

(5) To provide a novel and improved method which enables one todetermine the cause of non-equilibrium in a specific uranium and thoriumore;

(6) To provide a novel and improved method which enables one to analyzeuranium-thorium composite ores no matter what the equilibrium conditionof the ore may be.

Additional objects of my invention will be brought out in the followingdescription or a preferred embodiment of the same taken in conjunctionwith the accompanying drawings wherein,

FIGURE 1 is a graph depicting the gamma spectrum of an equilibriumuranium ore;

FIGURE 2 is a graph depicting the gamma-ray spec- FIGURE 5b is a graphdepicting the gamma-ray spectrum of the uranium group of nuclides;

FIGURE 5c is a view depicting the gamma-ray spectrum of the combineduranium and radium groups forming an equilibrium ore;

FIGURE 6 is a graph depicting the gamma-ray spec:- trum of anequilibrium uranium ore with the channels used in analysis indicated;

FIGURE 7 is a graph depicting the gamma-ray spectrum of an equilibriumthorium ore with the channels used in analysis indicated;

FIGURE 8 is a graph depicting the gamma-ray spectrum of a chemicallyrefined thorium oxide that is several years old;

FIGURE 9 is a graph depicting the gamma-ray spectrum of a sample ofmaterial from a deposit of the radium nuclides and their daughters ofboth the uranium and the thorium decay series;

FIGURE 10 is a graph depicting the uranium-thorium ratio as a functionof the ratio of gamma-ray energy emitted at key nuclides.

In its preferred form, my invention will be described principally as toits applicability to the assaying of radioactive ores such as uraniumand thorium bearing ores.

RADIOACTIVE ORE ANALYSIS BY GAMMA-RAY SPECTROSCOPY Radiometric methodsof analyzing naturally occurring radioactive ores have not been acceptedby industry as a means of analyzing ores on a routine basis because of alack of reliability of results. This lack of reliability in radiometricassays is due, largely, to a lack of radioactive equilibrium between thevarious nuclides in a decay series of a radioactive ore. If some methodcould be found to determine the equilibrium condition of an ore,accurate and reliable assays could be obtained by radiometric methods.

The present invention provides a simple and sure method for determiningwhether an ore is in a state of radioactive equilibrium or not; if not,the cause of the non-equilibrium can also be determined. Methods arethen presented by which equilibrium ores and certain non-equilibriumores can be assayed.

The particular radiometric method of the present invention consists inmeasuring the intensity of the gamma radiation of various energiesemitted by specific nuclides in a radioactive ore. To do this a devicecalled a gammaray scintillation spectrometer is used.

THEORY OF THE GAMMA-RAY SCINTILLATION SPECTROMETER The gamma-rayscintillation spectrometer consists of three principal units, ascintillation detector, a pulse height analyzer, and a sealer.

The scintillation detector When gamma radiation impinges on certaininorganic materials such as sodium iodide, the gamma photon is absorbedby the crystal; in the place of the absorbed gamma ray a flash of lightis emitted by the crystal. The intensity of this light in proportionalto the energy of the absorbed gamma ray. Optically coupled to the NaIcrystal is a photoelectric tube which converts the flash of light intoan electric impulse. The voltage or height of this impulse isproportional to the intensity of the light striking the cathode of thephotoelectric tube and thus proportional to the energy of the absorbedgamma ray. After proper amplification this voltage impulse is fed intothe pulse height analyzer.

The pulse height analyzer The pulse height analyzer is an electronicdevice that contains a gating circuit which can be set to receivevoltage pulses of a specific height. It accomplishes this by having twodiscriminators or thresholds. If the voltage of the impulse is not highenough to reach the setting of the lower discriminator it isdisregarded. If the voltage of the pulse exceeds the setting of theupper discriminator it is fed into an anticoincidence circuit which willcancel the output of the discriminator. Only pulses which exceed thesetting of the lower threshold but do not exceed the setting of theupper threshold are passed on to actuate a scaling mechanism. Thevoltage difierence between the upper and the lower setting of thediscriminator is termed the window width of the analyzer. The particularvoltage height that this window is set to intercept is termed a channel.

The scaler The scaler is another electronic device which simply countsthe output pulses of the pulse height analyzer discriminator. The countcollected by the scaler divided by the time over which the count 'wastaken is a direct measure of the intensity of the gamma radiation of aparticular energy being absorbed by the scintillation crystal.

The gamma radiation emitted by a partcular nuclide or isotope ismono-energetic, thus, the gamma-ray spectrometer channel can be set tocount the gamma rays coming from a particular nuclide. As the number ofgamma rays per unit time is proportional to the amount of that nuclidepresent, a quantitative analysis can be made of the nuclide. In thisway, by assaying an ore for key nuclides of the decay series, the stateof the L radioactive equilibrium of the ore can be determined.

NATURAL RADIOACTIVITY rays, or beta particles, are negatively chargedhigh speed 7 electrons. Gamma rays are very short wave lengthelectromagnetic radiations, the wave length of gamma radiation beingseveral orders of magnitude shorter than that of visible light. Bothalpha rays and beta rays are relatively easily absorbed in matter whichmakes the eflicient detection of these particles for ore analysis underroutine conditions often a difficult process. Gamma radiation, on theother hand, is not so easily absorbed; this malres detection of it underroutine conditions a more convenient operation.

A radioatom in decaying may emit more than one gamma ray or it may emitnone. In any case, no matter how many gamma rays are emitted, each gammaphoton will have a characteristic Wave length or energy. Table 1 andTable2 list the nuclides of the two major natural radioactive decayseries. Included in these tables are the gamma rays emitted by eachnuclide.

TABLE 1.THE URANIUM DECAY SERIES Energy of Name of Nuelide AtomicNuclide Half-Life Gamma Rays Number Emitted by Nuelide Grey.) 1

Uranium Group:

Uranium 1 92 U2518 4.5 10 y 47 (i). Uranium Xl 90 Th 24.1 d 93. UraniumX2- 91 P3 1.18 m 3952582, 806, Uranium IL.-- 92 U234 2.6Xl0 y 60, 93',11s. Ionium 90 Th 8.0 10 y 68,2130, 190, 228, Radium Group: V

Radium 88 Ram 1,600 y 188. Radon 86 Rn 3.825 11 Radium A 84 Pom 3.05 mRadium B 32 Pa 26.8 53, 242, 259, 295, Radium o 83 B1 17.9 m 636391,426, 49s, 1,120,1,238, 1,379, 1,520, 1,761, 1,820, 2,200, 2,420. RadiumC. 84 P 214 164 10- s... Radium D 82 Pb 7, 42;), 32, 37, 43,

, 65. Radium E 83 Bi Radium F 84 P0 84, 790. Radium G 82 Pb 1 Branchingdecays of less than 0.1 percent are omitted.

TABLE 2.-THE THORIUM DECAY SERIES Energy of Name of Nuclide AtomicNuclide Half-Lite Gamma Rays Number Emitted by Nuclide (key.) 1

Thorium Th 55, 75. Mesothorium 88 Ra 30. Mesothorium 2 89 Ac 60, 135,184, 338, 46 533, 590, 918, 969 Radiothorium 90 'Ih 87. Thorium X 88 Ra241. Thoron 86 Rn Thorium A 84 P0 Thorium B 82 Pb 115, 176, 238,

249, 299. Thorium C 83 131 40, 144, 164, 288, 328, 432, 452, 472, 720,830, 1,030, 1,340, 1,610, 1,810,

,200. Thorium O 84 Po 0.3 l0- s.-. Thorium C 81 T1 3.1 m 277, 510, 582,

Thorium D 82 Pbm Stable.

l Hollander, .T. M., Perlman, 1., and Seaborg, G. T., Tables ofIsotopes: Rev. Modern Physics, v. 25, pp. 469-651, 1953.

Radioactive decay The average time that a radioactive nucleus existsbefore it disintegrates into some other nucleus ranges greatly from onesubstance to another. The law that governs radioactive decay isexponential: consequently, it takes an infinitely long time for all ofthe atoms of any particular element to disintegrate. Because of this, aunit of time called the half-life is used to describe the rate ofradioactive decay of a particular nuclide. The half-life is the lengthof time it Will take for one-half of the initial atoms of a particularmaterial to disintegrate. Thus only one-half of a sample of U disappearsby radioactive decay in 4.5 X 10 years. In contrast to this, onehalf ofa sample of P0 disintegrates in 10- seconds. Nothing can be done in anyway to change the rate at which a given material will disintegrate. Thehalf-life is independent of pressure, temperature, and other conditionsunder natural control.

Radioactive equilibrium An atom that disintegrates to form another atomis called the parent and the product is called the daughter. Aradio-atom is in a state of secular radioactive equilibrium with itsdisintegration product when the same number of atoms of the daughternuclide disintegrate as are formed in a unit of time. Thus, in aradioactive. decay series in equilibrium, the number of atoms of anuclide being formed is exactly equal to the number of atoms of thatnuclide disintegrating. Therefore, the number of disintegrations perunit time is the same for each member in the decay series. As allmembers of the decay series do not decay at the same rate, there willhave to be a greater amount of the longer lived nuclides present toprovide the same number of disintegrations per unit time as that comingfrom the shorter lived nuclides: consequently, the amount of a nuclidepresent in a decay series in equilibrium is directly proportional to itshalf-life.

A state of non-equilibrium exists when all or part of one or more of thedaughters or parents is physically removed from the decay series. If anuclide with a short half-life is removed, equilibrium can be rapidlyregained. If a nuclide with a long half-life is removed, it may bemillions of years before complete equilibrium is regained between allthe members of the decay chain. After uranium is deposited it takesseveral millions of years to establish approximate equilibrium betweenall the members of the decay series mainly because of the long halflivesof two of the daughters of U A state of secular equilibrium, however, isregained between the first three members of the uranium grouprof'theuranium decay series in less than a year. And assuming that U has thesame chemical properties as U secular equilibrium is generallymaintained between the first four members of the uranium series. Atleast in nature this should be true. As all of the daughters of Ra haveshort half lives secular equilibrium is also maintained in this group ofthe uranium decay series when radium is present. When thorium isdeposited it takes only a few decades to establish approximateradioactive equilibrium between all of the members of the decay seriesas all of the daughters of Th have relatively short half-lives.

EQUILIBRIUM AND NON-EQUILIBRIUM IN ORE'S The various elements of a decayseries have varying chemical properties. Uranium is generally moresoluble in naturally occurring solutions than are the other elements ofthe uranium decay chain. If an ore deposit is exposed, and many of theColorado Pateau ores are at or very near the surface, the naturalprocesses of leaching can remove varying amounts of the valuable uraniumcontent of the ore having a deposit deficient in this element. As mostof the gamma radiation is emitted by the radium group of the uraniumdecay chain, such deposits can still be very radioactive.

A rather interesting process which will cause a state of non-equilibriumin ores is the migration of radon, a gas which is formed as the daughterof radium in the uranium decay series. It is not uncommon to find orebodies that vary in the state of equilibrium from the bottom to the topof the deposit because of the upward migration of radon. Such a processwould leave the bottom of the deposit deficient in the radium group ofnuclides, the center of the deposit in an approximate state ofequilibrium, and the top of the deposit with an excess of the radiumgroup of nuclides.

Another type of radioactive deposit is that in which only the nuclidesof the radium group of the uranium decay series are present. Manyprospectors have discovered abnormal radioactivity at well heads or inhot springs deposits. Generally associated with this activity is asulphate of barium or strontium. Radium, which has a halflife of 1600years, is taken into solution with these sulphates and when thesesolutions come to the surface, the radium is precipitated along with thesulphates. Because of the short half-lives of the daughters of radium,secular equilibrium between the parent and daughter products is rapidlyestablished. Inasmuch as some of the daughters of radium, notably Bi andPb, are copious emitters of gamma radiation, the deposit will appearvery radioactive. Seldom, however, do these ores contain uranium incommercial amounts.

In the uranium series the most important forms of nonequilibriumpresently recognized are:

(1) Radium group deficiency due to migration of radon;

(2) Daughter-product deficiency due to a lack of time to reachequilibrium or due to differential leaching of the daughter-products;and

(3) Uranium deficiency due to diiferential leaching of uranium ordeposition of daughter products only.

The key nuclides in a uranium ore, then, are U and Ra The gammaradiation emitted by these nuclides, however, is so weak it cannot bereadily resolved in a gamma spectrum of a uranium ore. A radiometricquantitative analysis is made, therefore, for nuclides which are copiousemitters of gamma radiation and which in nature are in a state ofsecular equilibrium with these key nuclides; the abundance of the keynuclides is then inferred from this analysis.

Gamma-ray spectra The gamma-ray spectrum of an equilibrium uranium oreis illustrated in FIGURE 1, while FIGURES 2, 3, and 4 illustrate thegamma-ray spectra of the most common types of non-equilibrium ores.

The method used to obtain the picture of the gammaray spectrum of an oreas illustrated in this application is not the conventional method usedin obtaining gammaray spectra curves with pulse height analyzers.Instead of sweeping the discriminator through the spectrum of voltagepulses by simultaneously lowering both voltage thresholds of thediscriminator so as to maintain a constant window width and produce alinear array of the gamma energy versus the intensity of gamma radiationat a particular energy,'the method used in the present invention was oneof holding the discriminator at a fixed level and then amplifying thevoltage pulses produced by the scintillation detector up through thediscriminator by increasing the voltage on the photomultiplier dynodes.Because the amplification of voltages in the photomultiplier is anexponential function of the applied voltage to each dynode, anexponential array of the gamma-ray energy versus the intensity of gammaradiation at a particular energy is obtained. Such a method is veryadvantageous because it condenses the spectra in the high energy regionwhere no important gamma-photon peaks occur while it spreads the spectraout in the low energy region where the gamma rays of interest inassaying occur. If at the same time the voltage pulses are beingamplified up through the discriminator, the output pulses of thediscriminator are fed into an ink recorder, a plot of the energy versusthe intensity of the gamma-radiation at a particular energy can be made.This was the method employed in producing the gamma-ray spectra graphsdepicted in the drawings.

URANIUM ORE ANALYSIS TABLE 3.OHANNELS USED IN COUNTING FOB. ORE ANALYSISApproxi- N uclides Primarily Nuclides Primarily Channel mate EnergyProducing the Producing the Number of Pea Peak in the U Peak in the T11(kev.) Series Series Threat 234 P m T11 5, Ta A022.

Ra T11 A0 238 Pb Pb 351 Pb Bi 969 B1 A0 The total count on any one ofthese channels does not come entirely from the nuclide primarilyproducing the peak. The resolution of the spectrometer is by no meanssharp and as Table 1 and Table 2 indicate there are a multitude of othernuclides emitting gamma rays with energies near that which the channelslisted in Table 3 are set to intercept. It may be assumed, however, thatthe majority of the gamma rays counted on a specific channel are fromthe nuclides indicated in Table 3. Backscattered and Compton efi'ectgamma rays may produce part of the low energy peaks, but as indicated inFIGURE 3 they have only a minor effect. Thus, With a uranium ore, thecount rate, expressed in counts per second, on channel 3is producedprimarily by Pb When thorium and uranium are present in the same ore thecount on channel 3 is produced by Pb of the uranium decay series and byPb of the thorium decay series. How much of the total count each nuclideproduces can be determined by a method outlined later.

With uranium ores in a state of radioactive equilibrium, the count rateof any channel is proportional to the amount of U 3 that is in the ore,

this value.

The equilibrium factor If the graphs of the spectra of the various typesof uranium ores are inspected it will be noticed that when uranium ispresent in the ore the count rate on channel 2 will'exceed that of thecount rate on channel 3. When the uranium is removed from an ore thecount rate on channel 2 will be less than that of channel 3. The ratioof the count rate on channel 2 to that on channel 3 can thus be used todetermine whether an ore contains any uranium ore or not. This ratio,the channel 2 count rate divided by the channel 3 count rate, is termedthe equilibrium factor. For uranium ores in a state of approximateradioactive equilibrium this ratio Will be between the limits 1.10 and1.30. If the uranium ore is not in a state of radio-active equilibriumthe value of this ratio will be outside these limits. By the value ofthis ratio the cause of the non-equilibrium can also be determined. Ifthe value of the equilibrium factor is greater than 1.30, the ore isuranium-rich. Generally, uraniumrich ores, those ores that consistprimarily of the uranium group of nuclides of the uranium decay series,will have equilibrium factors with values greater than 3.0. Radium-richores, those ores that consist predominatelyof the radium group of theuranium decay series, will exhibit equilibrium factors less than 1.00,the value of the factor being between the limits 0.85 and 0.98 in mostcases. Uranium-deficient ores, those ores that contain the radium groupof nuclides and certain nuclides of the uranium group, namely Th Pa andTh will exhibit equilibrium factors with values in the range 0.90 and1.00. Uranium-deficient ores are probably not very common in nature asH1 and Pa have relatively short half-lives and will decay to aninsignificant amount very rapidly without U to sustain them. This typeof ore is best exemplified by the tailings from a uranium concentrationprocess. Uranium-deficient ores can be distinguished from radium-richores as the count rate of channel 1 will exceed that of channel 2 foruranium-deficient ores, while the count rate of channel 1 will be lessthan that of channel 2 for radium-rich ores.

When thorium is present the value of the equilibrium factor will bebetween 1.10 and 0.30 depending on the amount of thorium that ispresent. For a pure thorium ore with no uranium decay series nuclidespresent the value of the equilibrium factor will be around 0.30. Amethod to determine how much thorium is present will be outlined later.

Analysis of uranium ores When the numerical value of the equilibriumfactor is in the range 1.10 to 1.30, indicating a state of radioactiveequilibrium for uranium ores and the presence of none of the nuclides ofthe thorium decay series, the percent of uranium in the ore can beobtained by multiplying the count rate on channel 2, C after anadjustment has been made for the background count rate on this channel,by a factor to convert from the count rate to percent uranium. Assays ofthis type are usually expressed as percent U the formula, therefore, isset up to yield Mathematically:

Percent of U O =x w g k C conditions, W g and k will be numericalconstants.

Therefore, u u u= 2m and Percent of U3O3= u 2U 2 The sampleself-absorption of gamma-radiation factor,

.x is a function of the amount of heavy elements in an ore and theabsorption coefficient of these elements. In the case of uraniumand-thorium ores, therefore, it is ain which e is the base function ofthe grade of the ore. As lead and uranium are the only heavy elements ofany consequence in a uranium ore, the self-absorption factor can beevaluated by means of the following type of expression:

of the natural logarithms, 2.718, ,u is adsorption coeflicient of leadand ,M is the absorption coeflicient of uranium, and t and t are thethicknesses of lead and uranium in the sample, respectively. Because thelead and uranium atoms are diffused throughout the sample it would bevery diflicult to evaluate this term. In any case, the amount of leadand uranium in the sample would have to be known before hand, which, ifit were, would eliminate the need for further analysis.

Fortunately, this correction is negligible for low grade ores, and inthe higher grade ores it can be evaluated by means of a graph containinga plot of the count rate on a particular channel versus the correctionfactor for the count rate of that channel. 7

Because of their general low grade, this factor will not enter theproblem with ores usually encountered in routine assaying in the UnitedStates. If the count rate of channel 2 is less than counts per second,therefore, this factor, x can also be considered a constant, the valueof which is 1.00.

For ores of a grade below about 5 percent U 0 then For the particulargeometry and sample weight used in the methods described the value of Kwas 0.0420.

The most prominent factors affecting the count rate obtained with aparticular sample on channel 2 and chanpercent of U O =K C nel 3 areones of sample-scintillation tube geometry, sample self-absorption, andsample weight. The absorption of the gamma radiation in the aluminumshielding of the crystal will be slight as the shielding is very thinand the absorption coeflicient of aluminum is small. In any case it willbe a constant percentage of each gamma ray of a particular energyemitted by the sample. The same is true of the aluminum cup in which thesample is held. The air between the sample and the crystal will haveeven .less eifect than the aluminum in absorbing the gamma radiationfrom the sample.

If the geometry of they crystal-sample set-up is held constant, thisfactor will introduce no counting variations for which adjustments mustbe made. Also if the weight of the sample being analyzed is held at aconstant value, no adjustments will have to be made for this factor. For

assays listed in this report the sample size was 20.0 grams.

The sample was placed in an aluminum cup 5.0 centimeters in diameter.The sample and cup were then cen tered on a cylindrical NaI crystalabout 2 millimeters from the free face of the crystal. The crystal usedin this research was -a 1 inch by 1 inch NaI(Tl) Harshaw crystaloptically mounted on 6292 Dumont photomultiplier tube. For the data usedto illustrate the methods outlined, a window width of 0.5 volts wasused.

If high grade ores are encounered, instead of making a self-absorptionadjustment the sample can be diluted before assaying to a grade below 5percent U 0 In most cases silica is a good material with which to dilutethe rich ores.

of distinguishing non-equilibrium type ores when they are encountered.

clides, C is greater than 0.40, the ore may be the formula will be setup to yield this Percent of ThO =x w g k C absorption, sample tubegeometry and unit-conversion factors, For a particular set of countingconditions T1102 361;,

Percent of ThO =K C (2) Remarks (Source of Ore) Utah.

Utah.

Utah.

Utah.

Utah.

Utah.

m'xtizywmash.

series nu analyzed for its thorium content by multiplying the channel 3count rate by a proportionality factor, K to con vert from count rateintensity to percent thorium. As the percent ThO value. MathematicallyWeight, samplerespectively.

and for ores of a grade below about 5 percent w g and k will benumerical constants, therefore: t t t t iiT, and

For the particular geometry and sample weight used 0. 407 Big IndianWash,

0. 834 Moab, Utah.

0.170 Standard Mines, Utah.

0.251 Blue Lizard Mine,

0. 224 Hidden Splendor M.,

0.21512 AEO Standard Ore.

0. 374 Big Indian Wash,

0. 452 Hidden Spender M.,

0.25 AEO Standard Ore.

1. 513 Lisbon Uranium,

4. 29 AEO Standard Ore.

1.103 Moab, Utah.

59 Cameron, Ariz.

0 93 Wellpi 0 48 Carlile,

0 94 Uravan, Colo.

0 18 Oarlile, Wyo.

1 02 Morocco, Africa.

5 percent of thorium in an ore is generally expressed as Where x w g,,and k are sample self- Chem.

Assay,

Percent Radio Assay, Percent Equi- Comp.

Factor 3/ 4 librium Factor,

Equil. Factor, 02/0 U 0 must be diluted or a EQUILIBRIUM FACTOR men-tmust be made. deficient ores are very difii- ROUTINE CHEMICAL ASSAYS nthis case, K is 0.021 instead Non-equilibrium uranium ores TABLE4.ANALYSIS OF URANIUM EQUILIBRIUM ORE BY FORMULA 1 TABLE 5.RECOGNITIONOF EQUILIBRIUM AND NON-EQUILIBRIUM ORES BY Of the non-equilibrium typeuranium ores, the uranium-rich ores can be assayed by gamma-raydiscrimination methods when the equilibrium factor is 3.0 or greater.The formula is the same as Formula 1, however, the

Radium-rich and uraniumcult to analyze with any accuracy because of theintense Table 5 is a list of three equilibrium and sixteennonequilibrium ores showing how the equilibrium factor is used toidentify each type of ore.

proportionality factor i of 0.042. Again, as in the case of equilibriumores, of a grade higher than 5 percent sample self-absorption adjustradioactivity of this type of material in comparison with the uraniumsuch an ore will contain; the uranium content being only a fewhundredths of a percent in most cases.

Ore No.

A-IB A46 THORIUM ORE ANALYSIS in the methods described, the value of Kwas 0.0403.

When the composite ore ratio of a radioactive ore 70 Same sample We 1ghtPP C geometry conhas a value of or greater, indicating a pure thoriumditions as were described in the analysis for uranium ores ore with noneof the uranium series of nuclides present, P Y the analysis 'f 9 Thereasol} for and the horium equilibrium factor, the channel 1 countcomputlng an extra thorlum eqmhbrlum fad)? 15 to check and see if it isonly the nuclides after 'I h that by the channel 3' count rate producedby the thorium 76 are present in the ore. The same processes that leachrate produced by the thorium series nuclides, C divided radium nuclidesfrom the thorium decay series and re deposit these nuclides elsewhere.As the first daughter of the thorium decay series, Ra has a half-life of6.7 years, such a deposit can remain quite radioactive for a number ofyears without 'Ih to sustain it. Some of the count rate on channel 1 isproduced by Th when it is not present in an ore the count rate on thischannel is lessened. A computation of the thorium equilibrium factorwill indicate this fact.

Because of the short-lives of all of the daughters of the thorium decayseries, the mother isotope of the decay series, Th will probably never,in nature, occur free of any of its daughters in any appreciable amountas will uranium. This fact is illustrated by FIGURE 8 which shows thespectrum of a refined thorium oxide which is but a few years of age.Notice the close similarity between this spectrum and that of a thoriumequilibrium ore as shown in FIGURE 7. This indicates that all thedaughters of the thorium decay series reach a state of approximatesecular equilibrium with their parents in a relatively short time.

As with uranium ores, the radium nuclides of the thorium series can beleached from the ore and redeposited elsewhere. This is indicated byFIGURE 9 which shows the spectrum of a material which contains theradium group of nuclides of both the thorium and uranium decay series.There is no measurable amount of Th or U present in this ore. It is thistype of ore that the thorium equilibrium factor is used to guardagainst.

Sample-self absorption of the gamma radiation becomes noticeable in oreswith a grade of ThO greater than about 1 percent, but it is negligibleuntil the grade of the ore reaches about percent ThO except forchannel 1. Gamma radiation on channel 1 is especially susceptible toabsorption in the heavy elements in the ore because of its very lowenergy. In place of an adjustment of the count rates, the ore can bediluted with silica to a grade below 5 percent ThO Also complicatingthorium assays is the fact that thorium seldom occurs free of uranium innature. This is indicated in Table 6 by the fact that the composite orefactor, C /C seldom has a value of 3.0 or above. Because of this,Formula 3 was used to calculate the percent of ThO of the ores listed inTable 6. Formula 3, however, reduces to Formula 2 when there is nouranium present in the ore.

The somewhat greater variance of the radiometric assay from the chemicalassay in the case of thorium ores can be partially explained by the factthat accurate chemi cal assays for thorium are very difficult to obtain.It may be, of course, that the radiometric assay is at fault. In anycase the results of Table 6 indicate that this sample-self absorptioncorrections should be made when the count rates of channels 1 and 2 goabove the values of 50 and 125 counts per second, respectively. Theother alternative is to dilute the sample until these count rates fallsbelow this value, the amount of silica added being carefully noted.

Composite uranium-thorium ores can be recognized by a uraniumequilibrium factor, C /C between the limits 1.10 and 0.30. Asradium-rich and uranium-deficient ores also exhibit equilibrium factorsin this range, an additional operation must be made to differentiate acomposite ore from these two non-equilibrium uranium ores. This can bedone by determining the count rate on channel 4 and calculating thechannel 3-channel 4 count rate ratio. This ratio is called the compositeore factor. For a pure uranium ore the value of this ratio isapproximately 0.62. This will apply whether the ore is uraniumdeficientor radium-rich or an equilibrium uranium ore. As uranium-rich oresproduce about the same count on channels 3 and 4, this type of materialwill have little effect on the value of the composite ore factor. Forthorium ores the value of this ratio is approximately 3.0. Values ofthis ratio between the limits 0.6 and 3.0 indicate uranium-thoriumcomposite ores.

In a composite ore the count rate on channel 3, C after subtracting thebackground count, is due to the sum of the count rate of the uraniumnuclides in the ore, C and the count rate of the thorium nuclides in theore, C Or, C =C +C Assuming both the uranium and the thorium to be in astate of radioactive equilibrium:

Dividing by K ThO yields:

0 en 3 5) I K34 rhozfKiT T1102) 1 and,

If the thorium series is in a state of radioactive equilibrium, theradiometric percent of ThO will be equal to the chemical percent of T110Because of the short half-lives of all of the daughters of the thoriumdecay series, near secular equilibrium usually does exist in thoriumores. In any case whether the thorium in the composite ore is in a stateof equilibrium with its daughsr T1102) Percent of 'ThO method should besufficiently accurate for routine laboraters or not can be determined bycalculating the thorium Y y equilibrium factor. Although thorium, atleast in nature,

TABLE 6.THORIUM ORE ANALYSIS RESULTS Equil. Comp. Thorium Radio Chem.Chem. Ore No. 01 C2 C3 C4 Factor, Ore Equil. Assay, Assay, Assay,

01/0 Factor, Factor, Percent Percent Percent 3/ 4 CIT/O31 T1102 T1102U301;

6. 09 17. 90 7. 73 0. 34 2. 32 0. 74 0. 0. 69 Tr. 42. 05 127.35 45. 000. 33 2. s3 0. 44 4. 88 5. 00 0. 20 4. 50 13. 65 4. so 0. 33 2. 85 0. 740. 52 0. 40 0.01 12. 51 34. 40 13. 72 0. 36 2. 51 0. 64 1.28 1. 44 0. 0548. 40 147. 00 51. 40 0. 33 2. 86 0. 49 5. 65 5. 36 0. 02 s. 07 24. 958. 12 0. 32 3.07 0. 65 0. 97 1.00 0.04 1. 65 4. 93 1. 7s 0. 34 2. 77 0.80 0. 19 0. 20 0.01 4. 00 12. 65 4. 15 0. 32 3.05 0.69 0. 50 0. 50 0. 026. 58 21. 23 6. 0.31 3. 17 0. 72 0. 35 0.80 0. 03

ANALYSIS OF URANIUM-THORIUM COMPOSITE ORES Uranium-thorium compositeores also can be analyzed by gamma-ray discrimination methods. Becauseit may will be in equilibrium with its daughters, the daughter productsof the thorium decay series, as illustrated by FIGURE 9, can existwithout being in equilibrium with their parent. The state of equilibriumof the uranium be difiicult to determine beforehand the grade of theore, 75 in the sample should have no effect on the thorium assay.

13 Equation 3 could not be solved if it were not for the fact that theuranium-thorium ratio,

can be plotted as a function of the composite ore factor,

C /C This relationship is shown in FIGURE 10. By

taking the ratio of the count rates of a composite ore on channel 3 andchannel 4 and by use of FIGURE 10, the value of the uranium-thoriumratio can be determined and substituted in Equation 3. For a solution toEquation 3 such a substitution is made and the percent T110 in the orecan be calculated directly. Multiplying this value of the percent Th inthe sample by the uraniumthorium ratio will yield the equivalent percentU 0 in the sample. That is the amount of uranium that will be in the oresample if the uranium group of nuclides is in equilibrium with theradium group of nuclides. As C comes mainly from the nuclides of theradium group of the uranium decay series, the assay for thorium is notaffected by any major state of non-equilibrium that might exist in theuranium part of the composite ore.

Before a uranium assay is determined, however, the equilibrium conditionof the uranium in the ore should be checked. This can be done bycalculating the count rate contribution of the thorium on channels 3 and2 and subtracting this value from the total count rate of thesechannels. The remaining values are the count rate contributions of theuranium nuclides on these channels. These values can be used in aconventional way to determine the uranium equilibrium factor F2 anuranium-thorium ratio. When the majority of the count on channel 2 isdue to the thorium series of nuclides it is best to use theuranium-thorium ratio method. This is because of the inability todetermine accurately the uranium count rate contribution on thesechannels when the uranium count rate contribution is small compared tothe thorium contribution. There is a fairly simple method by which todetermine which method to use: When the value of the uranium-thoriumratio is less than 0.20 use the uranium-thorium ratio method, and whenthe value of the uranium-thorium ratio is greater than 0.20 use theEquation 1 method. Both methods are illustrated in Table 7. When theuranium-thorium ratio is near 0.20 either method should yield about thesame result.

There are some combinations of uranium series and thorium seriesnuclides which would be diflicult to distinguish. One such combinationwould be the radium group of the thorium decay series with auranium-rich uraniumore. On chemical considerations, however, suchcombinations in nature are almost inconceivable. In any case if they doshow up the equilibrium factors, while they may indicate the wrong causeof the state of nonequilibrium, will at least indicate that the ore isnot in a state of radioactive equilibrium and therefore cannot beaccurately assayed by these methods. Chemical analysis should be used insuch cases.

The real value of this method is in the fact that the thoriumdetermination is independent of the equilibrium condition of the uraniumin the ore sample. As the equilibrium condition of the uranium can bedetermined, however, the method is valid for uranium assays. Table 7lists some results obtained in analyses made by using the methodsoutlined above When an assay is rejected the factor that led to itsrejection is underlined. A negative If the value of this ratio isbetween the limits 1.10 and 85 thorium equilibrium factor indicates onlythe radium TABLE 7.RESULTS OF ASSAYS ON COMPOSITE ORES Channel CountRates (c.ls.) (1) (2) Equilib- Compos- Thorium Uranium Radio. Chem.Chem. o N rium ite Ore Equil. Equil. Assay, Assay, Radio. Radio. Assay,

Factor, Factor, Factor, Factor, Percent Percent Assay, Assay, Percent 0C C 04 02/0 03/04 ClT/OBT CHI/0311 Th0: ThOz Percent Percent UaOa 41. 2024. 87 24. 28 34. 25 1. 02 0. 71 0.93 1. 11 0. 11 0. 10 1.06 1. 01 1.0044. 50 27. 21 28. 95 36. 40 0. 94 0.80 0. 73 1. 21 0. 34 0. 33 1.00 1.04 1. 00 18.18 13. 02 37. 55 13. 42 0. 35 2. 80 0. 41 1. 43 1. 44 l. 130. 10 0. 11 0.14 104. 78 73. 92 74. 94 103.03 0. 99 0. 73 49 1. 15 0. 570. 43 2. 96 2. 93 3.01 64. 05 35. 37 89.09 38.15 0. 2. 34 0. 58 1. 31 3.22 2. 68 0.43 0. 51 0.46 50. 70 30. 88 47. 20 41. 8O 0. 65 1. 13 0. 1.30 1.02 1.00 1.07 0. 99 1. 00 45. 27. 48 33. 20 37. 10 0.83 0.90 0.57 1.l1 0. 52 0. 50 0. 99 1. 00 1. 00 23. 20 12. 22 28. 85 12.89 0.42 2. 240.66 1. 48 1.03 1.00 0. 17 0. 20 0. 20 97. 52 122. 66 180. 94 195.03 0.0. 93 M (LL24 Nil 0.01 25. 32 18. 70 34. 94 23. 33 0. 54 l. 50 0.26 l.10 0 98 0.78 0. 51 0.49 0. 49 4s. 74 58.41 100. 76 89.47 0. 58 1.13 1.130 '75 Nil Nil 84. 61 53. 47 71. 29 70. 44 0. 1.01 0. 42 1. 15 1.331.07 1. 86 1.84 1. 82 20. 35 9. 51 26. 85 10.09 0. 35 2. 66 0. 68 1. 281.01 1. 00 0. 08 0. 10 0. 10 18. 43 8. 64 26. 15 9.02 0. 33 2. 0. 65 1.07 1.00 1.00 0. 06 0. 06 0.06

(1) Radio assay by U-Th method. (2) Radio assay by Equation 1 method.

1.30, the percent U 0 in the sample will be equal to an ar:-

To check the equilibrium condition of the thorium in the ore the countrate contribution of the uranium nuclides on channel 1 can be calculatedand subtracted from the total count rate of channel 1. The remainingcount rate will be due to the thorium in the ore sample and the thoriumequilibrium factor C 11 ar groups from the uranium and thorium seriesare present. D-9 and D-11 illustrate this type of ore. As D-9 and D-llcontain only the radium groups of nuclides from both series theradiometric assays for uranium and thorium will have no value. Theuranium equilibrium factor, C /C is a littl outside the limits set forequilibrium uranium ore in a few cases in Table 7. This is to beexpected when the uranium in a composite ore is a small percentage ofthat of the thorium. In such cases, personal judgement on the part ofthe analyst is important. In the case of ores D-3, D-8, and D-14 theauthor would say that the uranium is in equilibrium. The same is truefor the thorium equilibrium factor which in the case of D-4 even becomesnegative. Because of the small possibility of non-equilibrium thoriumores occurring with equilibrium uranium ores, the author also would saythe thorium in ore D-4 is in equilibrium with its daughters. The mainreason for these values wandering outside their limits is that it isvery difiicult to determine with any ac- 15 curacy the countratecontribution of the uranium or thorium nuclides on a specific channelwhen the largest share of the count rate on that channel is produced bythe other decay series. Especially is this true in the case of channel 1where self-absorption enters the picture in even low grade ores.

Table 8 lists the values for the various ratios and factors used inassaying composite and other uranium and thorium ores. The values of thefactors listed in Table 8 is a function of the efliciency of theequipment involved and the operating conditions of the pulse heightanalyzer. For example, a wider window width will decrease theproportionality factors. For each particular assaying setup with adifferent machine these factors will have to be calculated anew.Assuming equal resolution for all spectrometers used in this work, thenew factor will be related to the factor listed in Table 8 by somenumerical constant. The value of the uranium equilibrium TABLE 8.VARIOUSFACTOR VALUES AND LIMITS USED IN ASSAYING RADIOACTIVE ORES Name ofFactor Symbol Value Uranium Equilibrium Factor. 02/03 0.00-

A. For Equilibrium Ura- 05/03 1.10-1.30

nium Ores.

B. For Uranium-rich Ores. 02/03 300-100.

0. For Radium-rich Ores 01/03 0.70-0.95.

-D. Fqz) Uranium-Deficient 02/03 0.90-1.00.

res.

E. For Thorium Ores 01/03 0.30-0.35

F. Fig Thorium-Uranium 0 /03 0.30-1.10 res.

Composite Ore Factor 03/04 0.55-3.30 Thorium Equilibrium Fact0r Gin/Car0.40-0.95. Channel 1, Uranium Factor Km 38.3 c./s. per percent U308.Channel 2, Uranium Factor-.- Km 23.8 c./s. per percent UaOa. Channel 3,Uranium Factor Kw 20.5 c./s. per percent UaOa. Channel 1, ThoriumFactor.-. K1r 16.2 c.ls. per percent ThOz. Channel 2, Thorium FaetorKi'r 7.2 c./s. per percent T1102. Channel 3, Thorium Factor--- Ka'r 24.8c./s. per percent T1102.

factor, the composite ore factor, and the thorium equilibrium factorlisted in Table 8 will be the same for all gamma-ray scintillationspectrometers assuming the efliciency of all machines in detecting gammaradiation on the various channels bears a linear relationship.

The efficiency of a particular machine is a function of the size of thecrystal, the crystal-sample geometry, the size of the sample and theshape of the sample container. If these factors are reproduced asoutlined in this application all the factors listed in Table 8 shouldapply directly.

ACCURACY OF ASSAY VALUES It is difiicult to determine the absoluteaccuracy of the methods outlined in this application. The only standardores available are chemically assayed ores. The chemical assays used inthis application are in most cases routine assays and are notnecessarily exact. Some standard ores are available for purchase throughthe Atomic Energy Commission. The chemical assay of these ores is basedon a large number of independent assays from four differentlaboratories, yet the assay values given for these ores are stated to beaccurate to within only 2 percent of the given value.

The first fourteen of the chemical assays listed in Table 4 are theresult of very careful chemical analysis, being either Atomic EnergyCommission standard ores or ore pulps which were assayed by severalindependent agencies. The radiometric assay and the chemical assay agreevery closely in the case of these ores. The last six ores of Table 4were chemically analyzed on a routine basis. There is a decided increasein the variance of the radiometric assay and the chemical assay.Considering the small variance of the radio assay and the carefullychemically analyzed samples, there is some justification for assumingthat it is the routine chemical assays that are at fault in the case ofthese last six ores.

Inasmuch as most uranium assaying is done by the .fluorimetric method,mention might be made of the accuracy attainable by this method. Thefollowing is quoted from a United States Geological Survey Circular, No.199, With careful work uranium may be determined by the fluorescencemethod with no greater error than 4 percent of the uranium content.Under routine conditions, where speed may be important, the errorgenerally is greater and may amount to: plus or minus 8 to 15 percent ofthe uranium content. When errors occur the results are generally low.This seems to indicate that under routine laboratory onditions anaccuracy of plus or minus 10 percent of the actual uranium contained inthe sample is good. The accuracy of the assaying methods suggested inthis application certainly seems to fall within this range.

A nuclear disintegration is a random phenomenon which is subject to theestablished methods of statistical analysis. A mathematical analysis ofthe statistical fluctuations involved in scintillation counting,however, is difficult at best. This is especially true when pulse heightanalysis is involved, as the statistical fluctuations can arise fromseveral sources in the conversion of gamma radiation into photons oflight in the scintillator, in electron emission at the photocathode, inelectron multiplication at each dynode, in pulse amplification, and inpulse height discrimination in the analyzer.

To reduce the possibility of error in the assay results due to anincorrect determination of the true desintegration rate of an ore, aslong count periods as possible should be used. Too long count periods,however, defeat one of the main reasons for using radioactive assays,that of speed in obtaining analysis results. By gathering a total ofabout 7500 counts on each channel the probable statistical error inobtaining the true count rate of the ore is in the order of one percent.

When the grade of the ore being assayed was about 0.4 percent U 0 orgreater, 7500 counts were gathered on each channel. When the grade ofthe ore fell below this value a smaller number of counts was gathered sothe total assaying time for any one sample did not exceed ten minutes.The accuracy of the result is liable to be reduced in doing this,however, with ores of a grade of 0.10 percent U 0 or less an accuracy oftenor twenty percent of the contained uranium is suflicient for routineassays.

Table 9 contains a list or" the results of a succession of independentassays on a single sample of an ore indicating the reproducibility ofresults with this method. The larger the count gathered, the moreaccurate the radiometric assay. The ore used in gathering the data forTable 9 was a carefully chemically assayed Colorado Plateau ore. Thechemical assay was 0.603 percent U 0 The average values of theradiometric assays and the mean deviation for the various length ofcounting runs is also listed in Table 9.

CONCLUSIONS The methods described in this application for analysis ofradioactive ores seem to be sufficiently accurate for routine oreanalysis. The speed and simplicity of radiometric methods can thus beused to great advantage.

Another significant feature of these methods is their reliability; whenit is impossible to assay an ore accurately this fact is indicated.Chemical assaying could be used on these few samples. A majority of theores normally encountered in routine assaying, however, can be assayedby these energy-discrimination methods. The saving in time is apparent.Important also is the fact that highly skilled technicians are notrequired to do the analyzing. The use of chemical reagents is notinvolved in any part of these procedures.

There is an additional saving in time and labor in sample preparation,as the sample need not be finely ground. Crushing to one-quarter inch issuflicient. Weighing of the sample to a tenth of a gram is all that isrequired, thus, this part of the process, time-consuming in the chemicalmethod, can be markedly shortened.

TABLE 9.REPRODUOIBILITY OF RADIOMETRIO ASSAYS Count Rates (c./s.)Average Standard Equilib- Radio. Value of Devia- Run No. Time per riumAssay, Assays, tion of Assay Factor, Percent Percent Assays, C2 C3 0210sUaOs UaOs Percent M-l 9 minutes--. 14. 34 12. 40 1. 16 0. 602 M- -do 14.33 12.34 1. 16 0. 602

The fact that a much larger sample is being used 1n key nuclide toobtain a value indicative of equilibrium in assaying ores by this methodmeans that a more representative sample of the whole is being assayed.The analysis that results by this method is an assay of the completesample under the scintillometer tube. In chemical assaying seldom morethan a gram of ore is used for the analysis. With thisenergy-discrimination method a 20 or 50 or IOO-gram sample can be used.Thus the sample being analyzed is much more representative of the whole,and thus more reliable results can be obtained.

It seems apparent, then, that these methods should be valuable forroutine laboratory assaying. On the other hand the equipment involved islight enough for field use. More important, as these methods can yield acontinuous and instantaneous assay they can be adapted for use in millcontrol.

Another important implication of this method is that of being able toassay ores of elements which although not naturally radioactive can bemade artificially radioactive through neutron bombardment. Preliminaryexperiments indicate that ores of copper and gold can be assayed byactivating a sample of such an ore with neutrons and then measuring thegamma radiation emitted when the excited atoms disintegrate.

Theoretical considerations indicate that a large num* ber of othermaterials, such as silver, cobalt, manganese, and tin can also beassayed by activating such materials by neutron bombardment andmeasuring the characteristic gamma ray that will be emitted by theactivated atom.

I accordingly do not desire to be limited in my protection to thespecific ores or specific details illustrated and described except asmay be necessitated by the appended claims.

I claim:

1. The method of determining equilibrium in uranium and thorium orescomprising measuring the gamma ray radiation given oil by key nuclidesnormally expected to be present in such ore, and dividing the gamma rayradiation intensity from one key nuclide by that flom another such ore.

2. The method of determining equilibrium in a uranium ore comprisingmeasuring the gamma ray radiation given oif of the key nuclide thorium230 and the gamma ray radiation given oii by the key nuclide lead 214and dividing the intensity of radiation from the key nuclide thorium 230by the intensity of radiation from the key nuclide lead 214 to obtain avalue indicative of equilibrium in such ore.

3. The method of determining equilibrium in a thorium ore comprisingmeasuring the gamma ray radiation given oif by the key nuclide thorium232'and the gamma ray radiation given 011? by the key nuclide lead 212,and dividing the intensity of radiation from the key nuclide thorium 232by the intensity of radiation from the key nuclide lead 212 to obtain avalue indicative of equilibrium in such ore.

4. The steps in the method of assaying composite ores of uranium andthorium comprising measuring the gamma ray intensity at gamma ray energyof approximately 238 kev. and the gamma ray intensity at gamma rayenergy of approximately 351 kev., dividing the 238 kev. gamma rayintensity in the 351 kev. gamma ray intensity to obtain a count rateratio and determining from said count ratio, the ratio of uranium tothorium in such ore.

References Cited in the file of this patent UNITED STATES PATENTS Paul:Nuclear Geology, John Wiley and Sons, Inc., New York, 1954, pages250-255.

Upson et al.: Analyzing for Low-Energy Gamma Emitters in a RadionuclideMixture, Nucleonics, vol. 13, No. 4, pages 38-42, April 1955.

4. THE STEPS IN THE METHOD OF ASSAYING COMPOSITE ORES OF URANIUM ANDTHORIUM COMPRISING MEASURING THE GAMMA RAY INTENSITY AT GAMMA RAY ENERGYOF APPROIMATELY 238 KEV. AND THE GAMMA RAY INTENSITY AT GAMMA RAY ENERGYOF APPROXIMATELY 351 KEV., DIVIDING THE 238 KEV. GAMMA RAY INTENSITY INTHE 351 KEV. GAMMA RAY INTENSITY TO OBTAIN A COUNT RATE RATION ANDDETERMINING FROM SAID COUNT RATIO, THE RATIO OF URANIUM TO THORIUM INSUCH ORE.